Sorry it took so long, I got a cold or whatever that just laid me out. Anyway, I’m alive again. So a 753 is a linkage machine. I wasn’t sure when I mentioned the solenoid bit. There is a direct mechanical connection between your levers and the valving on the hydrostat. I’d start with making sure those linkages are set up right. Then just cuz I’m me, I’d double check that charge pressure. At that point, since you’ve replaced the drive motor, there really isn’t anything left but the hydrostat itself.
I had a Gehl in once that had bizarre drive problems. It turned out to be a control arm that had a broken weld inside the valve block on the hydrostat. I had to remove the valve block, take it apart, re-weld the control arm, and Bob’s your uncle. That was a new one on me.
If you haven’t done so already, get the book. They usually have a good troubleshooting section at least that might help.